Vortex-antivortex behavior in a tilted YBa2Cu3O7-delta crystal

Previous magnetization-vector measurements on a YBa2Cu3O7-delta crystal, ti lted by angles (theta) of up to 90 degrees relative to the fixed field appl ied initially along the c axis during cooling to 4.2 Ii, have revealed that the original trapped vortices along c develop sizeable branches in the ab planes. These measurements have now been extended on the same crystal to ti lt angles of up to 180 degrees, and it is found that B-ab, the flux density component along ab, reverses in sign and reaches a large negative maximum near theta = 135 degrees, despite the positive orientation of the internal field along nb. This anomalous behavior of B-ab is attributed to the entry of antivortices that form branches antiparallel to the original vortex bran ches along nb as a consequence of strongly attractive antivortex-vortex int eractions. The varying coexistence of antivortex and vortex branches is see n to be reversible as theta is cycled between 90 degrees and 180 degrees. ( C) 1999 Elsevier Science B.V. All rights reserved.